Geoelectrical study of middle Miocene-recent sequences southwest Karbala city, central Iraq

The study area is characterized by arid climate, covering some 160 Km2 of dissected Middle Pleistocene alluvial fan and lies 6 Km southwest of Karbala City. Tectonically, it lies along Abu Jir Fault on the western margin of the Mesopotamia Foredeep. This work aims to study and analyze the Middle Miocene – Recent stratigraphic sequence utilizing Vertical Electrical Sounding (VES) in order to determine the possible presence of groundwater aquifers. The resultant type curves include a variety of earth models, ranging from 3 to 8 layers. Two contour maps of groundwater are presented with average aquifer depth of 38.1 m, and average resistivity of 11.4 Ωm. Analysis of the electrical anisotropy coefficient for the sequences overlying the aquifer indicates that both the resistivity – anisotropy, and thickness – anisotropy relationships follow polynomial functions. Furthermore, the anisotropy coefficient is more sensitive to resistivity variation than bed thickness variation. The occurrence of water saturated clay beds and marls (i.e. high conductivity) within a stratigraphic sequence render the electrical medium anisotropic. The map of the electrical anisotropy indicates that the high values in the northern half of the study area may be attributed to the presence of claystone beds. A lack of gypcrete alteration in this area, coupled with low topographic relief supports the hypothesis that the flow of rain water saturates the underlying claystone beds. Results of drilling show the aquifer to be confined, and overlain by impermeable dry claystone beds, of relatively high resistivity. However in the northern region, the shallow (more porous) upper units of the same claystone sequence are water saturated (by rainwater infiltration) giving rise to similar low resistivity values to the aquifer. It is concluded that the geoelectrical models are highly affected by the tectonic and structural setting of the study area, as indicated by the depth offsets of vertically contiguous resistivity responses. In addition, the marl beds, which underlie the aquifer bearing zone, exhibit a negligibly small resistivity contrast with the aquifer, such that in many models its response is integrated with the aquifer as a single electrical layer. The resultant isopach maps of aeolian sand (top soil) and gypcrete indicate the eastern and southeastern parts of the study area to be most useful for agricultural development.